1. Field of the Invention
The present invention relates to transmission line length measurement method and apparatus for measuring the length of an open-ended transmission line in accordance with a reflected waveform obtained by suppling a pulse to the transmission line, and especially relates to transmission line length measurements and apparatus suitable for high precision measurements of the length of a transmission line from circuits connected to these test terminals or pins for outputting test patterns and for judging test results to input/output terminals of tested circuits in an integrated circuit test apparatus having a plurality of test terminals or pins.
2. Description of the Prior Art
The length of each of the transmission lines from circuits connected to these test pins for outputting test patterns and for judging test results to input/output terminals of circuits under test varies from one line to another or among the test pins.
In a timing calibration, which is required for maintaining test timing accuracy in an integrated circuit test apparatus, it is one of the major procedures to calibrate the timing error caused by a deviation of the transmission line length between the test pins and input/output pins of the circuit under test.
One conventional method for measuring the length of a transmission line is disclosed in Japanese Laid-open Patent Application No. 176560/1983. In this prior art, as shown in FIG. 1, the time difference between a leading edge formed by the incident wave end a leading edge formed by the reflected wave of a stepwise wave obtained by superposing the incident wave with the reflected wave when the far end of the transmission line is opened is measured. This time difference corresponds to the transmission delay time when a signal is transmitted through the transmission line back and forth, and therefore, it is assumed that a half of this time difference corresponds to the net length of the transmission line.
One of the major factors determining the transmission delay time of a transmission line is a transmission velocity component determined by distributed capacitance and distributed inductance of the transmission line. If the transmission line exhibits no loss and no distortion in propagating waveforms over the frequency range of the input signal, the input signal can propagate through the transmission line without distortion of its waveform. In this case, the net propagation delay time coincides with one half of the time difference between a leading edge formed by the incident wave and a leading edge formed by the reflected wave of a stepwise wave obtained by superposing the reflected wave with the incident wave when the far end of the transmission line is opened.
However, an actual transmission line in a practical use, for example, such as coaxial cables and micro strip lines, contains loss components and a limited frequency band as well as group delay distortion, which are dependent on the materials and structures of the transmission line. Furthermore, in an actual setup of connections between the integrated circuit test apparatus and devices under test, the input terminals and the packages of the devices under test have capacitance components, so that the high frequency component of signals may be attenuated due to the time constant component determined by that capacitance and the characteristic impedance of the transmission line. These actual conditions affect the signal waveform from the output terminal of the transmission line and accordingly the signal waveform is distorted in response to the input waveform.
In the actual transmission line described above and usually used, the attenuation of the high frequency component of the signals is a significantly dominant factor, and if the pass frequency band of the transmission line is narrower than the frequency band of the input signal, the transient change in rise and fall behaviors of the output signals is less steep. Ordinarily, the transmission delay time of the transmission line is defined by measuring the time difference between the input signal and the output signal at their center value of the signal amplitude and hence, in distorted waveforms of the output signals, the extended time in the rise and/or fall behavior is substantially included in this time difference to be measured. The rise time and/or fall time increases as the length of the transmission line is extended but the relationship between the time and the length is not proportional. Assuming that the loss of high frequency components in a wave is mostly caused by the skin effect, the increase of the rise time and/or the fall time can be represented by an error function of the length of the transmission line. Assuming that the loss of high frequency components is mostly caused by the capacitance and the characteristic impedance of the transmission line, the increase of the rise time and/or the fall time can be represented by an exponential function of the time constant defined by the capacitance and the characteristic impedance of the transmission line. Accordingly, if the length of the transmission line including such a loss from the reflected waveform is measured as one-half of the time difference between the incident wave and the reflected wave mentioned above, the measured length of the transmission line contains an error from the proportional relationship of the increase of the delay time with the transmission line length.
For example, when a signal having a 100 ps leading time propagates by 1 m through a coaxial cable having a 10 dB/10 m loss at 1 GHz, the simple estimation of the transmission delay time is a few ps greater than the exact transmission delay time. If the same signal is mentioned above propagates by 30 cm through a micro strip line having a 3 dB or more loss at 3 GHz or less, the error between the simple estimation end the exact delay time is greater than 10 pico seconds. The longer the rise time of the input signal, the fewer the high frequency component in the input signal, so that the error between the simple estimation and the exact delay time is likely to be reduced. However, in semiconductor devices provided to operate at several hundreds MHz, their rise time is as small as 100 pico seconds, and in the conventional form of connections between the integrated circuit test apparatus and devices under test, it may be a frequent occurrence that a transmission line having worse transmission performance than that described above is used. In order to precisely test semiconductor devices with operating frequencies at several hundreds MHz, it is required that the time difference among a plurality of test signals supplied to devices under test and the time difference among judging circuits for judging a plurality of output signals from the devices under test are calibrated on the order of pico seconds. In the above mentioned prior art, however, Such a precise measurement of the transmission line length cannot be realized.
Conventionally, in measuring the transmission line length, such an apparatus as disclosed in Japanese Laid-open Application No. 150877/1989 is used. An example of prior art apparatus will be described with reference to FIG. 2.
In FIG. 2, two phase difference detecting circuits 101 end 102 are used. Input terminals A1 and B1 of the phase difference detecting circuits 101 end 102 are commonly connected to one end of a transmission line 100 to be tested with its far end open. A pulse signal S1 having a constant repetition period is supplied to the input terminals A1 and B1, and a reference pulse signal S2 having a constant repetition period which is slightly different from that of the pulse signal S1 is supplied to the input terminals A2 and B2. In the phase difference detecting circuit 101, the phase difference between the input signal at the terminal A1 end the input signal at the terminal A2 is compared at the timing of the first leading edge of the stepwise reflected wave at the input terminal A1. In the other phase difference detecting circuit 102, the phase difference between the input signal at the terminal B1 and the input signal supplied to the terminal B2 is compared at the timing of the second leading edge of the stepwise reflected wave at the input terminal B1.
In FIG. 2, the phase difference information signals S3 end S4 from the phase difference detecting circuits 101 and 102 are supplied to an EOR(Exclusive OR) gate 103, and its gate output S5 and the reference pulse signal S2 are supplied to an AND gate 104. The AND logic gate 104 outputs the reference pulse signal S2 as a valid pulse signal S6 only when the phase difference S3 and S4 do not coincide with each other. By counting the number of the valid pulse signals S6 by the counter 105, the time difference between the first leading edge and the second leading edge of the stepwise reflected wave is obtained, end one-half of the time difference thus obtained is used as the transmission delay time of the transmission line when a wave travels one way in the transmission line.
As a state where the phase difference information signals S3 and S4 from the phase difference detecting circuits 101 end 102 do not coincide with each other, there exist two states, i.e., one state where the leading edges of the phase difference information signals S3 and S4 do not coincide with each other and the other state where the trailing edges of the phase difference information signals S3 end S4 do not coincide with each other. For example, in case of detecting the phase difference at the leading edges of the input signals A1 and B1, the leading edges of the phase difference information signals S3 end S4 occur at the time that the leading edges of the input signals A1 and B1 cross the leading edge of the reference pulse signal S2. On the other hand, the trailing edges of the phase difference information signals S3 and S4 occur at the time that the leading edges of the input signals A1 end B1 cross the trailing edge of the reference pulse signal S2.
Both of the phase difference between the leading edges of the phase difference information signals S3 and S4 end the phase difference between the trailing edges of the phase difference information signals S3 and S4 are determined by the transmission delay time 2T when a wave travels back and forth along the transmission line, the repetition period t of the pulse signal S1, and the repetition period t+dt of the pulse signal S2. Each of the phase differences is equal to 2T (t+.DELTA.t)/.DELTA.t under an ideal condition.
However, the actual phase difference detecting circuit has its own inherent characteristics of detection sensitivity. The more its detection sensitivity is lowered, the more the detected phase difference contains errors.
Therefore, if there is a difference in the detection sensitivity characteristics of the phase difference detecting circuits 101 and 102 between detection between the leading edges and detection between the leading edge and the trailing edge, the detection result in the phase difference between edges with lower detection sensitivity characteristics contains a greater error. In the above mentioned conventional measurement method, this error is inevitably included in the measured value.
For example, when a circuit disclosed in Japanese Laid-open Application No. 233382/1988 is applied to the above-mentioned phase difference detecting circuit, the phase difference between the leading edges can be detected with the detection sensitivity of a few pico seconds according to current semiconductor technologies. In contrast, the detection sensitivity in the case of the phase difference detection between the leading edge and the trailing edge is low in principle, so that there is a problem in obtaining an accurate measurement.
In the above described conventional technologies, the number of reference pulse signals repeated during one period in which both of the phase difference information signals do not coincide with each other corresponds to an amount equal to twice the length of the transmission line. Accordingly, it is required that the number of reference pulse signals repeated be counted during one period in which both of the phase difference information signals do not coincide with each other, or to precisely determine how many periods in which both of the phase difference information signals do not coincide with each other corresponds to the counted number of reference pulse signals. However, there is no means for realizing such functions other then a supplemental means for controlling the number of input signal pulses to the phase difference detecting circuits.